Add small conv YAML validation

[guosran]

Summary

Addresses #292 by adding a small conv validation case. This PR adds the focused small YAML test and generator support needed to reproduce the conv control/dataflow issue without running the large 4200-iteration test.

Changes

• Add validation/test/conv/conv_small.yaml.
• Add cgra/test/CgraRTL_conv4x4_test_from_yaml.py.
• Extend validation/script_generator.py for the small conv YAML flow.
• Add shared const opcode definitions used by the generated control stream.

Validation

CGRA_CONV_NI=2 CGRA_CONV_NJ=2 CGRA_CONV_YAML=validation/test/conv/conv_small.yaml \
python3 -m pytest cgra/test/CgraRTL_conv4x4_test_from_yaml.py::test_homogeneous_4x4_conv_combinational_mem_access -q --tb=short

[guosran]

This adds a small conv YAML/test path so we can validate the split RTL fixes without running the very large 4200-case test.

CGRA_CONV_NI=2
CGRA_CONV_NJ=2
CGRA_CONV_YAML=validation/test/conv/conv_small.yaml

Validation command:
python3 -m pytest cgra/test/CgraRTL_conv4x4_test_from_yaml.py::test_homogeneous_4x4_conv_combinational_mem_access -q --tb=short